Raw is not log, log is not raw. They are very different things.

Having just finished 3 workshops at Cinegear and a full day F5/F55 workshop at AbelCine one thing became apparent. There is a lot of confusion over raw and log recording. I overheard many people talking about shooting raw using S-log2 or people simply interchanging raw and log as though they are the same thing.

Raw and Log are completely different things!

Raw simply records the raw, unprocessed data coming off the video sensor, it’s not even a color picture as we know it, it does not have a white balance, it is just digital “1’s” and zeros coming straight from the sensor.

S-Log, S-Log2, LogC or C-Log is a signal created by taking the sensors output, processing it in to an RGB or YCbCr signal and then applying a log gamma curve. It is much closer to conventional video, in fact it’s actually very similar, like conventional video it has a white balance and is encoded into colour. S-Log etc can be recorded using a compressed codec or uncompressed, but even when uncompressed, it is still not raw.

So why the confusion?

Well, if you tried to view the raw signal from a camera shooting raw in the viewfinder it would look incredibly dark with just a few small bright spots. This would be impossible to use for framing and exposure. To get around this a raw camera will convert the raw sensor data to conventional video for monitoring. Many cameras including the Sony F5 and F55 will convert the raw to S-Log2 for monitoring as only S-Log2 can show the cameras full dynamic range. At the same time the F5/F55 can record this S-Log2 signal to the internal SxS cards. But the raw recorded on the AXS cards is still just raw, nothing else, the internal recordings are conventional video with S-Log2 gamma (or an alternate gamma if a look up table has been used). The two are completely separate and different things and should not be confused.

UPDATE: Correction/Clarification. OK, there is room for more confusion as I have been reminded that ArriRaw uses Log encoding. It is also likely that Sony’s raw uses data reduction for the higher stops via floating point math or similar (as Sony’s raw is ACES compliant it possibly uses data rounding for the higher stops). ArriRaw uses log encoding for the raw data to minimise data wastage and to squeeze a large dynamic range into just 12 bits, but the data is still unencoded data, it has not been encoded into RGB or YCbCr and it does not have a white balance or have gain applied, all of this is added in post. Sony’s S-Log, S-Log2, Arri’s LogC, Canon’s C-Log as well as Cineon are all encoded and processed RGB or YCbCr video with a set white balance and with a Log gamma curve applied.

9 thoughts on “Raw is not log, log is not raw. They are very different things.”

I agree they are not the same thing. But I would dispute “very similar to conventional video”. Log has many of the benefits of raw, as this demo video for my FCP X LUT plugin illustrates – http://bit.ly/12tUSs2

While the theoretical ideal would always be to record the pure unmodified 16-bit linear sensor output, some form of compromise is often necessary. Both ARRIRAW and REDCODE RAW use log coding internally as a means of data rate reduction. Sony do not publish how their raw works, but if they use no use any log coding as part of their compression they are missing a trick. Any lossy compression needs to remove information the loss of which will be least noticed.

Linear uses half of the available code values for the brightest stop, which is very inefficient. In 16 bit linear you are using 32768 levels to code the variations in the highlights right up against clipping. This is not where the important image detail is, and the vast majority of those levels will be merged in the grade anyway. Log coding uses the same number of code values per stop. At 10-bit, Cineon log is 90 code values per stop, which is not really enough but was hidden by film grain. 14-bit log can code 16-bit linear with no meaningful loss, but I would challenge anyone to see the difference in the end result between 16 bit linear and 12 bit log, particularly considering that the last couple of bits in a 16 bit A/D will be well into the noise floor.

But no one I’m aware of is recording 14 bit log. The ACES workflow does use data reduction for the higher stops using rounding above bit 2048 and as the Sony raw is designed to be compliant with this I suspect Sony use data rounding to minimise data wastage as for the brighter stops you can afford to have less data precision. But this is still very different to log as there is still a lot more data allocated to the brighter stops. You must remember that the top stop may be using a lot more data with raw, but that’s simply because it does contain a lot more picture information. It is our own visual system the perceptively thinks there is no more information in a brighter stop than a darker stop, the reality is different and this is important when manipulating an image. Many, many people get bitten by this when they over expose log, putting skin tones too high up the log curve. When you grade the levels back down the skin tones remain squashed together and while they can be stretched back apart by de-logging etc, they still never look as they should because the log compression means the in between values were never actually recorded and some of the important subtleties of the skin tones are lost. Log is very good for what it is designed for, capturing a bigger range where perceptually we don’t notice the data reduction in the highlights, but over expose log and it will bite you. Raw is far more forgiving and you can push linear raw far more than log.

It is possible Sony are using some bit rounding to get better data distribution, this would have very little effect on the image, less so than log encoding. This is how ACES linear works and Sonys raw is fully ACES compliant.

Do you have a response to Nick’s point: Red and Arri do use a log-encoded raw format. The way it’s worded sounds like its not just truncating the noisy bits. Surely there’s a reason they’re recording raw to a perceptual rather than linear format?

Thank you for your comments. I learn so much from hearing these conversations between intelligent folks.

Arri still use 12 bit log raw. It works well, but times are moving on and it’s now really pretty easy to record 16 bit data. Red used to use log, but now use 16 bit linear. The big thing about recording 16 bit linear is you simply take the data direct from the sensor and record it with zero processing. Why discard picture information when these days there really is no need?